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Known to increase lean body mass and bone density in the elderly, human growth hormone has been further linked to aging with a finding that it activates a gene critical to healing.

Researchers from the University of Illinois at Chicago say that the link between human growth hormone and the gene, Foxm1b, sheds light on the aging process.

"Growth hormone levels decline as we grow older; as a result, the Foxm1b gene stops working and our bodies are less capable of repairing damage," says Robert Costa, professor of biochemistry and molecular genetics.

Cell stimulation

Human growth hormone is released by the pituitary gland and is necessary for normal skeletal growth in humans.

"The literature had suggested that growth hormone therapy in elderly men stimulates cells to divide," says Costa. "We wanted to find out how the hormone worked at a molecular level."

Growing livers

In one study, Costa and colleagues inserted Foxm1b into aged mice whose livers had been partially removed.

The gene is involved in the entire life cycle of mammalian cells—their proliferation, maturation and death.

Its activity is elevated in dividing cells in young mammals and diminishes in old age.

The experiment showed that the gene restored levels of Foxm1b proteins and induced the livers to grow back at a rate typical of young mice.

Further research detailed how the gene coordinates cell activity and encourages cell division and multiplication.

Youthful regeneration

The researchers have now tested whether Foxm1b activity and youthful regeneration can be restored by growth hormone.

To this end, they removed parts of the liver in aged mice and then injected them with human growth hormone.

Tests showed that Foxm1b activity increased dramatically, as did levels of various regeneration-related enzymes and proteins.

In mice receiving growth hormone, the liver regenerated at a rate comparable to young mice, with cell proliferation peaking in just two days and full liver restoration occurring in one week.

In mice that didn't receive the hormone, the liver took up to a month to regenerate.

Essential component

Without growth hormone to activate Foxm1b, the researchers found, the gene remains stuck at the low level of activity found in old age, with liver cells failing to multiply rapidly enough for a speedy recovery.

While growth hormone injection is a controversial antiaging therapy, and several studies have shown that prolonged use has side effects ranging from diabetes to carpal tunnel syndrome, Costa believes that short-term use could be valuable for speeding repair after injuries or surgery in the elderly.